The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the acquisition of NMR data using rapid gradient echo pulse sequences such as spoiled gradient echo sequences.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1, is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. Whereas the more conventional pulse sequences have repetition times TR which are much greater than the spin-spin relaxation constant T2 so that the transverse magnetization has time to relax between the phase coherent excitation pulses in successive sequences, the fast pulse sequences have a repetition time TR which is less than T2 and which drives the transverse magnetization into a steady-state of equilibrium. Such techniques are referred to as steady-state free precession (SSFP) techniques or xe2x80x9csnapshotxe2x80x9d techniques. One such pulse sequence is spoiled, gradient-recalled echo pulse sequence.
While some xe2x80x9csnapshotxe2x80x9d techniques purposefully acquire data when he magnetization is in a transient state, data acquisition during steady state is often preferred in order to avoid blurring and image artifacts caused by fluctuating view-to-view NMR signal levels.
The standard method for assuring that magnetization has achieved steady-state prior to data collection is to repeat the imaging pulse sequence many times without acquisition of the NMR signals. As many as 200 of these so-called xe2x80x9cdummy repetitionsxe2x80x9d or xe2x80x9cdisdaqsxe2x80x9d may be required to drive the magnetization into a steady-state condition for some pulse sequences.
While the few extra seconds wasted prior to data acquisition are inconsequential for many common clinical applications, several applications would be significantly improved if this time could be reduced. Three such applications are: (1) real-time interactive imaging, where changing the excited slice disturbs magnetization equilibrium; (2) ECG-triggered cine imaging, where pausing the sequence between heartbeats disturbs magnetization equilibrium; and (3) centrically acquired angiography sequences where acquisition timing with respect to contrast arrival may be critical.
The present invention is a preparatory pulse sequence for an MRI system which rapidly drives magnetization to a prescribed steady-state level prior to the acquisition of NMR data using a snapshot imaging pulse sequence. More particularly, the preparatory pulse sequence includes applying an RF pulse which tips the longitudinal magnetization of spins being imaged by a flip angle xcex2; waiting for the magnetization to recover to a prescribed level during a recovery interval Trec; and then acquiring NMR data using a prescribed snapshot imaging pulse sequence.
A general object of the invention is to rapidly drive spin magnetization to the steady-state level for a prescribed snapshot image cquisition. By applying an RF pulse to the spins, their longitudinal magnetization is quickly driven below the desired steady-state level and begins to rapidly recover at a rate determined by the spin relaxation time T1. The recovery interval Trec is selected such that this spin magnetization recovers to the steady-state level as the data acquisition is begun using the prescribed snapshot imaging pulse sequence